Operators of trucks driven on public highways must typically monitor the weight of the truck payloads to remain in compliance with the legal load limits and avoid paying substantial fines associated with violating such limits. Monitoring the payload weight is often a difficult task when the payload is placed upon the truck at a remote location, such as a logging site, construction site, gravel pit, or other location where truck scales are not easily accessible. In these situations, it is desirable to have an onboard means for measuring the weight of the payload placed upon the truck or a trailer towed behind the truck.
Various devices have previously been used to measure the weight of a payload placed upon a truck trailer. Such devices include a single point suspension scale 3, such as the one illustrated in FIG. 1. The scale may take the form of a bending beam load cell or a double-ended shear beam load cell. As shown in FIG. 1, the scale 3 is typically mounted between a frame attachment member 25 which extends downwardly from the trailer frame 11, and a frame mounting bracket 31. The frame mounting bracket 31 is attached to a conventional trunnion tube 7 which retains springs 12 and axles 14. One drawback with these scales is that they often must be welded directly to the trailer frame attachment member 25 and to the frame mounting bracket 31 at weld joints 17. The weld joints 17 are subjected to high stress as a result of the payload weight and harsh environmental conditions, such as mud, rain, sleet, and ice. As a result, the welded joints 17 tend to fail, putting the trailer out of service. The failed joints must be repaired before the trailer may be returned to service, an expensive operation made more expensive if the failure occurs in a remote environment.
A further drawback of these prior art scales is that, due to high torsional forces induced by frame bending and twisting, it has been difficult to design scales that ignore these false torsional forces and instead respond only to the vertical load force. For example, experience has indicated that simply driving the vehicle to another spot on the road has caused the scales to report a false zero shift in signal output due to torsional forces induced by the frame of the vehicle, resulting in an erroneous weight measurement. Further, these types of scales have been found to be quite sensitive to off level loading conditions. For example, if the vehicle is loaded while being positioned on a 5 to 15 degree slope, the accuracy of the weight measurement degenerates.
Another drawback of these prior art scales is that they are interposed between the trailer frame and the conventional trunnion tube 7, raising the frame above the trunnion tube. In order to avoid increasing the overall height of the trailer, a frame retrofitted with such a scale must be shortened in the region of the scale to accommodate installation of the scale. This requires cutting the frame attachment member 25, to remove the required material. This is not only an expensive and time-consuming operation but also degrades the structural integrity of the vehicle. The operation is necessary if the scale is to be welded in place, as shown in FIG. 1, and is also necessary if the scale is a replaceable unit. For example, if the scale is to be bolted to the frame attachment member 25 and the frame mounting bracket 31, material must still be removed from the attachment member and/or the mounting bracket, and mounting plates sized to receive the scale mounting bolts must be welded in place. Whether the scale is replaceable or non-replaceable, it is difficult for truck owners to accept the fact that cutting critical suspension support members of the vehicle is required for these types of onboard scale installations.
A further drawback of the present scales is that in the event of a failure of the scale itself, the scale is not easily replaceable in the field. Because the scale 3 is welded to the trailer frame attachment member 25 and to the frame mounting brackets 31, replacement of the scale requires breaking the welded joints 17 between the scale and the frame attachment member, and between the scale and the frame mounting bracket 31, and re-welding a new scale to both the frame attachment member and the frame mounting bracket. Such operations are expensive and time consuming and particularly difficult if performed in the field.
Accordingly, there is a need for an onboard truck or payload weight measurement device which does not suffer from the problems associated with existing devices. The present invention fulfills this need and provides further related advantages.